The present invention relates generally to optical scanner systems and more particularly to an optical scanner system for creating a flying spot linear trace of a laser light beam to scan a straight line at an output plane in space. The invention finds particular use in the field of laser beam scanner systems as are used for reading originals on a copy board by a read laser beam and directly transferring the read information to a write laser beam to simultaneously expose a photosensitive surface of a plane to produce a printing plate.
In the art of printing plate technology increased use is made of electronic methods for recording, storing and/or generating information such as by computers, cathode ray tubes, facsimile devices and the like. For example in U.S. patent application Ser. No. 805,445, filed June 10, 1977 now abandoned in favor of Ser. No. 941,763 filed Sept. 13, 1978, entitled OPTICAL SCANNER AND SYSTEM, there is shown an optical scanner apparatus including a rotating pyramid scanning wheel with reflector segments interposed to deflect a laser beam into and out of a doublet mirror roof reflector associated with said scanner wheel. The scanner wheel introduces components of vertical and horizontal angular and translational deviation into the beam. The vertical angular deviation component is cancelled by inversion through the roof reflector while the horizontal angular component is doubled upon the second reflection from the scanner wheel so that the output beam is vertically wobble free and stable while the beam is scanned through the sum of the horizontal angular components introduced by the wheel. The scanner is employed to create a flying spot scan from a laser beam in a photosensitive plate exposure apparatus, one form of which employs superimposed laser read and expose (write) beams of different frequencies which are simultaneously scanned without chromatic aberration, and subsequently separated to read copy and to expose a photosensitive plate surface as in the production of printing plates.
The scanner consists of the roof mirror assembly comprising a first and second mirror disposed relative to each other to define a reflective doublet into which the beams of the read and write laser are deflected by the scanner wheel which is optically coupled to the roof mirror assembly. The scanner wheel has a plurality of planar segments facets arranged in a pyramid shape which progressively rotate about an axis of rotation through the common path of the read and write laser beam causing the same to be deviated.
The beams are deflected by the facets for successive reflection to each surface of the roof mirror assembly one time and for receiving the output of the roof mirror assembly for a second reflection by the same facet from which the beams were first reflected. After the second reflection the beams emerge through a scanner output opening spaced between the first and second mirror of the roof mirror assembly.
The facets are tilted with respect to the axis of the beam path so as to introduce a component of angular horizontal deviation which shifts the beams simultaneously and progressively from one side to the other across the original and plate surface while introducing no component of vertical angular deviation after the second reflection.
In the described system, the read laser beam is scanned over the original at a read platen or copy board to develop a signal which changes in response to sensed reflections from the original. The signal controls a modulator in the write beam laser beam path so as to control its output. The write and read laser beams are superimposed along a common laser beam path and are delivered to the scanner. The scanner introduces a horizontal, angular scan component into the beams while introducing no up and down or vertical angular component. After leaving the scanner, the beams are passed through a field flattening objective lens and are separated by suitable optics for being passed to the respective read platen and exposure platen.
The scanner wheel takes the shape of a triangular regular pyramid cut from a circular disc. Its sides form a plurality of planar reflector segments. The wheel is mounted and driven for rotation about an axis symmetrically passing through the apex of the pyramid so as to bring each segment in turn through the path of the laser beam and to deflect the same by reflection at an angle of tilt of one half of the pyramid angle, which angle causes the planar segment to vary its angles on an orientation input-output reflection surface to the scanner. As each segment rotates through the beam path its varying angular orientation introduces horizontal and vertical components of angular deviation into the beam. The horizontal angular components are doubled upon the second (output) reflection from the wheel segment while the vertical component is cancelled by inversion through the roof reflector doublet. More specifically, as each segment passes through the beam path it is disposed at an angle of tilt with respect to the axis of rotation which is equal to one half the apex angle of the associated pyramid and therefore the segment presents an angle of reflection to the beam which varies through the angle of tilt from side to side. The angle is a maximum when the normal vector of that segment is in alignment with the beam. The angle of tilt changes to a vertical minimum on each side. Simultaneously, the horizontal angle introduced is at a maximum at each side of the segment and is null at the middle. In this way the beam is deviated through twice the horizontal angular component introduced and thence delivered to the field lens for focusing onto the respective planes.
Also, in a system such as disclosed in U.S. patent application Ser. No. 522,103 filed Nov. 8, 1974, now abandoned, entitled LASER READWRITE SYSTEM FOR THE PRODUCTION OF ENGRAVINGS, there is shown an apparatus for producing an exposed photo plate from a copy board paste-up. A laser scanner system having a read laser beam is focused to a spot scanned across the copy board in a predetermined pattern, such as a raster-like scan. The reflection from the copy board is sensed, read and used to control the intensity of a second laser beam via a modulator. The second laser beam is used to impinge upon and scan a photosensitive surface. Both the read laser beam and the write laser beam are scanned through deflection optics and subsequently separated to impinge upon and focus at the copy board and photosensitive surface respectively. In this way there is a resultant exposure of the photosensitive surface in correspondence with the copy. As shown in Ser. No. 522,103 the scanner optics employed utilize a moving mirror galvanometer. Both the read and write laser beams are aligned and superimposed upon each other through suitable beam combining optics and are passed through the galvanometer simultaneously and subsequently separated by suitable beam deflection optics to the respective planes. Another optical system shown therein employs a polygonal scanner wheel having a plurality of surfaces directed outwardly from an axis of rotation of the wheel. The surfaces serve to scan the read/write beams through an angle and thereby create a flying spot scan.
In U.S. Pat. No. 4,081,842 entitled FACSIMILE SYSTEM there is disclosed a variation of laser read/write apparatus in which a facsimile system is developed. As disclosed therein, a duplication of read and write equipment at separate locations can be coordinated to form a facsimile transmission system. At the read station an optical scanner detects the input copy with the scanner spot, and the reflected light produces a video read data signal detected through a spatial mask to provide a transmitter video reference which gates a video read data before transmission. In the receiver a second optical scanner of similar construction is controlled by a video write data signal. The video write data signal gates a scanner spot of exposure laser beam light on and off to expose the output photosensitive copy surface at the receiver. Additionally, the scanner light is detected through a further spatial mask to provide a receiver video reference signal utilized to form a video write signal. The spatial masks in the transmitter and receiver have a known relationship or are identical so that the scanning of the output copy in the receiver can be spatially synchronized with the scanning of the input copy in the transmitter. As therein disclosed, each of the scanner optics includes a galvanometer operated mirror for scanning the incident laser beam back and forth through a horizontal angle.
The foregoing instruments as described in U.S. patent application Ser. No. 522,103, now abandoned, employ a field flattening lens for causing the beam provided from the scanner device to be focused at the plane of the copy board and photosensitive surface respectively and are known therefore as flat bed scanners. The scanner optics using a polygonal drum require very close tolerance during the manufacturing processes so as to control facet to facet tilt. Any error in facet to facet orientation together with bearing run-out errors and the like contribute to produce an angular or positional error component normal to the scan line which has come to be known as wobble or vertical error. Accordingly, the polygon design is expensive to produce due to the tolerances required and the facet to facet error has to be removed by some suitable means which is itself termed a "dewobbler" so as to remove the remaining facet errors.
The resonant or oscillating galvanometer scanner, as described in U.S. Pat. No. 4,081,842, pivots the galvanometer mirror in a sinusoidal manner. Since the scan velocity is non-linear, the more linear center portion of the scan is utilized. In this case, it is necessary to scan back and forth in opposing directions in order to maintain an efficiency level of approximately 50% scan efficiency. Such scanning requires lag compensation accomplished by deviating the read beam as a function of system time delays and scan velocity. Since such lag compensation is largely due to beam position in the write modulator, it creates a system in which the lag can be only partially removed at additional cost. If multiple machines are to communicate in a facsimile system a great deal of calibration of each machine is required to normalize the amount of lag produced in each machine. Lag error or for that matter any error in the facsimile process when scanning in both directions results in left writing and right writing images that are no longer superimposed, resulting in severe image degradation for even small errors. Accordingly, such back and forth opposing direction scan is limited. Further, with the demand for higher speeds particularly associated with facsimile systems, the scanner requirements go beyond galvanometer mirror performance capabilities related to the physical stability and support of a galvanometer mirror when operated at high resonant oscillation speeds since its internal structure must withstand back and forth movement at increasingly high torque.
While other systems exist utilizing curved fields which are cylindrically oriented, such are also limited in the scan efficiency. In one such system for example using a spinner type scanner in a cylindrical configuration, one scan is accomplished for each rotation of the scanning device. When exposure times are computed for a standard format, extremely high rotational speeds are required and synchronization of facsimile versions is difficult. Furthermore, such curved field systems require that the exposure surface be adaptable to a curved conformation which is often incompatible with printing plate production.